Acids and Bases in Organic Chemistry

Learning Objectives

• Define Brønsted-Lowry acid, Brønsted-Lowry base, conjugate acid, and conjugate base.

• Use curved arrow notation to show the flow of electrons in acid-base reactions.

• Use pKa values to compare acidity and basicity, and predict the position of equilibrium.

• Compare acidity and basicity of compounds based on structural analysis, including conjugate base stability and the ability to stabilize negative charges (ARIO).

• Explain the significance of the leveling effect and solvating effect.

• Differentiate between Lewis acids/bases and Brønsted-Lowry acids/bases.

• Describe the roles of carboxylic acids and amines in amino acids, and identify the zwitterionic form.

Brønsted-Lowry Acids and Bases

Definitions

• Brønsted-Lowry acid: A species that donates a proton (H+).

• Brønsted-Lowry base: A species that accepts a proton.

• Conjugate acid: Formed when a base accepts a proton.

• Conjugate base: Formed when an acid loses a proton.

Example: The reaction between hydrochloric acid and water:

• Here, HCl is the acid (proton donor), H2O is the base (proton acceptor), Cl- is the conjugate base, and H3O+ is the conjugate acid.

Curved Arrow Notation in Acid-Base Mechanisms

Electron Flow Representation

Organic reactions involve the movement of electrons. Curved arrows are used to show how electron density shifts during bond making and breaking.

• Curved arrows indicate the movement of electron pairs.

• In acid-base reactions, the base uses a pair of electrons to attack the acid and form a new bond, while the acid loses a proton.

• All acid-base reactions occur in a single step, with simultaneous bond breaking and making.

Example: The base attacks the acid, forming the conjugate acid and conjugate base:

Quantitative and Qualitative Analysis of Acidity

Using pKa Values

The strength of acids and bases can be measured quantitatively using pKa values, and qualitatively by analyzing the stability of their conjugate bases.

• Quantitative analysis: Uses numerical pKa values to compare acid strength.

• Qualitative analysis: Considers structural features that stabilize or destabilize the conjugate base.

Acid Dissociation Constant:

• Lower pKa = stronger acid.

Each pKa unit represents a tenfold difference in acid strength.

Comparing Acidity and Basicity

Conjugate Base Stability (ARIO)

The stability of the conjugate base determines the strength of the acid. The more stable the conjugate base, the stronger the acid.

• ARIO factors:

  1. Atom: Type of atom carrying the charge (size and electronegativity).

  2. Resonance: Delocalization of charge increases stability.

  3. Inductive effects: Electron-withdrawing groups stabilize negative charge.

  4. Orbital: The type of orbital holding the charge (more s-character = more stable).

Example: Comparing the acidity of ethanol and acetic acid:

• Acetic acid is more acidic than ethanol because its conjugate base is resonance stabilized.

Position of Equilibrium in Acid-Base Reactions

Predicting Equilibrium

The direction of acid-base equilibrium favors the formation of the weaker acid and weaker base.

• Compare pKa values: The side with the higher pKa (weaker acid) is favored.

• Alternatively, compare the stability of the conjugate bases.

Example:

• If of HA < of HB, equilibrium favors products.

Leveling Effect and Solvating Effect

Leveling Effect

Water can act as both an acid and a base, limiting the strength of acids and bases that can be used in aqueous solution.

• Acids stronger than H3O+ cannot exist in water; they are leveled to the strength of H3O+.

• Bases stronger than OH- cannot exist in water; they are leveled to the strength of OH-.

Solvating Effect

The ability of a solvent to stabilize ions affects acidity. Steric hindrance can reduce solvation and thus decrease acidity.

• For example, tert-butanol is less acidic than ethanol because its conjugate base is less well solvated due to steric hindrance.

Lewis Acids and Bases

Definitions and Comparison

• Lewis acid: Accepts a pair of electrons.

• Lewis base: Donates a pair of electrons.

• All Brønsted-Lowry acids/bases are also Lewis acids/bases, but not all Lewis acid/base reactions involve proton transfer.

Example: (Lewis acid) reacts with (Lewis base) to form .

Acids and Bases in Biological Systems

Amino Acids and Zwitterions

• Amino acids contain both an amino group (basic) and a carboxylic acid group (acidic).

• At physiological pH, amino acids exist as zwitterions, with both positive and negative charges.

• This dual nature is crucial for protein structure and function.

Selected Ammonium Ions and Their pKa Values

Table: Ammonium Ions, pKa Values, and Conjugate Bases

Additional info: Table entries inferred from standard organic chemistry data for ammonium ions.